Method for the electrolytic deposition of chromium and chromium alloys

ABSTRACT

The invention relates to a method for depositing chromium and/or chromium alloys on metals, and particularly on sheet steel, wherein an alloy layer is electrolytically deposited on the metal from a solution containing chromium ions and/or chromium ions and further metal ions such as Zn, Cd, In, Pb, Bi, Mo, Cu, Fe, Ni, Co, Mn, Al, Sb, Ag, Sn, Mg, wherein chromium hydroxide precipitated from a chromium (III) solution is used for regenerating the electrolyte solution using chromium (III) ions.

FIELD OF THE INVENTION

The invention relates to a method for the deposition of chromium andchromium alloys on metals, in particular on steel sheets.

BACKGROUND OF THE INVENTION

EP 0 285 931 A1 has disclosed a corrosion-resistant coated steel sheetand a method for manufacturing it. In the method according to this priorart, a zinc/chrome-based alloy, which contains a quantity of more than 5wt. % but no more than 40 wt. % chromium and whose remainder is composedof zinc, is deposited on at least one side of the steel sheet. In themethod, zinc ions and trivalent chromium ions or a mixture of trivalentchromium ions with a mixture of at least one metal from the iron familyand zinc are deposited from an acidic electrolyte solution. Thissolution should contain approximately 10 g/l-150 g/l of zinc ions, 10g/l-100 g/l of chromium ions, and 10 g/l-100 g/l of ferrous metal ions.The content of zinc and chromium ions here should be from 0.2 mol/l-3mol/l. The anions are either sulfate or chloride ions, with the additionof a complexing agent and an antioxidant. In order to deposit azinc/chromium alloy layer that contains more than 5 wt. % of chromium,it should be necessary to maintain the zinc content and the chromium ioncontent in the coating solution at a requisite high level. If trivalentchromium ions in the form of chromium sulfate or chromium chloride areintroduced into the coating solution, this increases the sulfate orchloride content, which interferes with the coating process. Trivalentchromium ions cannot be introduced in the form of chromium oxide ormetallic chromium here because these substances are not soluble, even atvery low pH values. Trivalent chromium ions could be introduced into thecoating solution in the form of chromium hydroxide or chromiumcarbonate. Both compounds, however, are unstable and transform intoinsoluble chromium oxide as they age.

In the method according to EP 0 285 931 A1, the trivalent chromium ionsare introduced by mixing metallic zinc and an aqueous solution ofhexavalent chromium with the acidic coating solution, which containszinc ions and trivalent chromium ions. In this case, metallic zincdissolves and the hexavalent chromium ions are transformed intotrivalent ones.

This method has the disadvantage of requiring the use of hexavalentchromium solutions. But the use of hexavalent chromium has been sensiblyrestricted for environmental reasons.

JP 08 246 198 A has disclosed adding chromium ions to a correspondingcoating solution by reducing Cr (VI) with hydrogen peroxide. This, too,requires the use of hexavalent chromium, which is unacceptable.Moreover, the use of hydrogen peroxide results in a powerful reactionthat is too dangerous for use on an industrial scale.

JP 80 91 842 A has therefore disclosed using organic reagents to reduceCr (VI) ions to Cr (III) ions, but this is also unacceptable due to thepresence of Cr (VI) ions in the process. In addition, the electrolyte ismixed with undesirable organic reagents.

JP 08 049 099 A has disclosed electrolytically dissolving metallicchromium in a process that requires destruction of the passive filmthrough mechanical contact with zinc, magnesium, or aluminum. Thedisadvantage here is that this is particularly expensive. In summary,the prior art either uses Cr (VI) or carries out an enrichment withother elements that likewise have a negative impact on the electrolyticcoating.

DE 10 2006 035 871 B3 has disclosed a method for depositing chromiumlayers in the form of a hard chromium plating, an electroplating bath,hard chromium plated surfaces, and their use. In this case, an ammoniumchromium sulfate, chromium chloride, and chromium sulfate are added tothe electroplating bath. The reference mentions that a metered additionand removal of sulfuric acid is required; chromium must be removed fromthe electrolyte during the chromium plating and subsequently dissolvedin the form of a sulfate or chloride. The enrichment with sulfate inthis case must be prevented through complicated separation of the anodechamber and cathode chamber with continuous removal of acid (pH valueregulation). The resulting constant rise in sulfate concentration leadsto the complete replacement of the bath after a certain time.

DE 195 23 307 A1 has disclosed a chromium plating method using trivalentchromium in which a trivalent chromium-containing chromium plating bathand an electrode are used. The trivalent chromium should be selectedfrom the group comprised of chromium (III) sulfate, chromium (III)chloride, chromium (III) oxalate, chromium (III) carbonate, and chromium(III) hydroxide. It should, however, be noted that chromium (III)hydroxide is not commonly used and is not even available on the marketbecause it is not stable. In addition, DE 195 23 307 A1 mentions thattrivalent chromium chloride is added to chromium; also, the testing ofthe pH value with sodium hydroxide does not hinder the enrichment ofchlorides in electrolytes.

DE 35 30 223 C2 relates to an acidic bath for galvanic depositing ofalloys of chromium with at least one of the metals iron, nickel, orcobalt; chromium (III) salts are also mentioned here, but not chromiumhydroxide. This reference also does not disclose a solution to theproblem of sulfate enrichment.

DE 26 57 012 C2 relates to a galvanic chromium bath; the chromiumplating in this prior art is carried out as a piece-by-piece chromiumplating, with the baths being replenished with the required reagentsuntil they are no longer to be used and are discarded completely. Thisis not possible with continuous processes, where the concentration ofanions and cations must remain constant.

DE 24 57 582 C3 relates to an aqueous, acidic galvanic chromium bathbased on chromium (III), containing trivalent chromium, bromide ions,ammonia ions, and acid-containing anions of complexing substances. Forexample, the chromium can be added in the form of chromium (III)chloride or chromium (III) sulfate, using chromium tanning liquor, e.g.in the form of a three-percent alkaline chromium sulfate solutionobtained by reducing sodium dichromate with sulfur dioxide as a suitablechromium source. Other suitable salts include chromium formate oracetate.

The object of the invention is to disclose a method for electrolyticallydepositing chromium and chromium alloys on metals, which is notproblematic in terms of environmental protection and workplace safetyand reliably permits continuous depositing of chromium and chromiumalloys with no negative impact on quality.

SUMMARY OF THE INVENTION

The method according to the invention is described by way of examplebelow in the context of a deposition of a zinc/chromium alloy on a steelsheet:

As explained above, for the deposition of zinc/chromium, it is necessaryto continuously replenish the electrolyte with zinc and chromium duringproduction in order to assure uniform production results. In this case,due to the acidic pH value, zinc can easily be dissolved metallically inthe electrolyte. Metallic chromium does not dissolve in the acidicelectrolyte; Cr (III) ions are required for the deposition, though.There is, however, no suitable Cr (III) compound that dissolves well inthe electrolyte and does not introduce foreign anions as in the priorart. Chromium oxide does not dissolve well and is not suitable. Chromiumsulfate does in fact dissolve, but enriches the electrolyte withsulfate; in the case of alkaline chromium sulfate, a sodium enrichmentalso occurs. Chromium (VI) compounds can no longer be used for reasonsof workplace safety and environmental protection. Reducing CrO₃ withhydrogen peroxide, which is also known, is too dangerous.

According to the invention, chromium hydroxide (Cr(OH)₃) freshlyprecipitated from Cr (III) ions is used to replenish the electrolytesolution. Cr(OH)₃ is not stable and after a certain amount of time,transforms into insoluble chromium oxide. In order to be able to controlthis reaction, with the method according to the invention, zinc oxideand possibly a liquor (ammonia, caustic soda solution, and the like)is/are added to a solution containing alkaline chromium sulfate oranother water-soluble chromium (III) compound such as chromium potassiumsulfate, ammonium chromium sulfate, chromium chloride, chromium acetate,chromium nitrate, and the like to produce a cake composed of zinc oxideand chromium hydroxide. The zinc oxide provides the increase in pH valuerequired for the precipitation, as well as a good filterability of thefilter cake.

In case of a deposition of chromium and chromium alloys without zinc,the addition of zinc oxide is omitted and the pH value is raised usingother suitable substances.

This precipitate of zinc oxide and chromium hydroxide is separated outfrom the precipitation solution directly after the precipitation and isdissolved in the electrolyte thus raising the zinc and chromiumconcentration, but avoiding the formation of insoluble chromium oxide.In a simple and easy-to-control way, this introduces chromium (III) ionsfrom the original chromium (III) solution into the electrolyte withoutalso introducing an undesirable anion.

In this case, it is advantageous that the chromium (III) compound useddoes not have to be of high purity since the precipitation processpurifies the product. Alkaline chromium sulfate is an easy-to-obtainmass-produced product. The sodium that it contains in addition to thesulfate likewise does not negatively impact this process since itremains in the filtrate and is completely separated out by theprecipitation.

A typical alkaline chromium sulfate with an alkalinity of 33% has thefollowing typical composition in terms of oxides:

24%-26% Cr₂O₃,

25%-27% SO₃,

22%-25% Na₂SO₄

22%-25% H₂O.

BRIEF DESCRIPTION OF THE DRAWING

The invention will be explained by way of example in conjunction with adiagram; the sole FIGURE is a very schematic flowchart of the process.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In a first reaction vessel 1, for example a catch basin, water, zincoxide, and alkaline chromium sulfate are mixed; the initial solution istaken to represent 100% portions of zinc, chromium, sodium, and sulfateions. This mixture is appropriately stirred and the precipitationreaction occurs. The supernatant solution and lime sludge are fed via apump 2 to a filter system. The sludge can also be separated from thefiltrate by means of sedimentation, centrifugation, or anothersolid/fluid separation method. The lime sludge and the supernatantsolution are separated in the filter system, the filtrate—i.e. thesupernatant solution—is separated out and of the initial quantity of100% each, still contains 24% zinc, the 0% chromium, 100% sodium, and65% SO₄. The resulting filter cake, after a single washing, contains 76%zinc, 100% chromium, 0% sodium, and 25% SO₄ as compared to the initialsolution. The sulfate quantity can be further reduced through additionalwashing of the cake. The resulting filter cake is added to theelectrolyte vessel 4 or more precisely, to the electrolyte, and isdissolved therein, if need be with the aid of mechanical devices.

The dissolution of the filter cake in the electrolyte can occur invessels that do in fact communicate with the electrolyte vessels inwhich the actual deposition is carried out, but are spaced apart fromthem and situated in such a way relative to the supply line diameterthat the dissolution of the filter cake does not interfere with thedeposition. It is then possible to provide pumps between such adissolving vessel (not shown) and the actual electrolytic bath so that acontinuous exchange takes place, for example in the form of a flowcircuit. The required replenishment of Cr (III) ions during thedeposition is determined and the filter cake is produced in acorrespondingly discontinuous way, but if possible, is furnished in sucha way that after a first filter cake is dissolved, the next respectivefilter cake is ready to be dissolved, but does not yet show any signs ofaging due to its storage time.

The washed filter cake is prevented from drying. This makes it possibleto also bridge interruptions of the production process without aging ofthe precipitated chromium hydroxide.

An advantage of the invention is the fact that with an acceptably lowequipment cost and the use of commercially available, easily obtainablechemicals, a continuous replenishment of electrolyte solutions for thedeposition of chromium and chromium alloys is achieved, which is notproblematic in terms of environmental protection and workplace safety.

1. A method for depositing chromium and/or chromium alloys on metals andparticularly on steel sheets, comprising: electrolytically depositing analloy layer on the metal from a solution containing chromium ions aloneand/or chromium ions and other metal ions selected from the groupconsisting of Zn, Cd, In, Pb, Bi, Mo, Cu, Fe, Ni, Co, Mn, Al, Sb, Ag,Sn, and Mg, and using chromium hydroxide precipitated from a chromium(III) solution to replenish the electrolyte solution with chromium (III)ions.
 2. The method as recited in claim 1, wherein the time between theprecipitation of the chromium hydroxide from the chromium (III) solutionand the introduction of the precipitated chromium hydroxide into theelectrolytic solution is calculated so that the chromium hydroxide doesnot transform into a poorly soluble product.
 3. The method as recited inclaim 1, comprising using an aqueous chromium (III) compound selectedfrom the group consisting of alkaline chromium sulfate, chromiumpotassium sulfate, ammonium chromium sulfate, chromium chloride,chromium fluoride, chromium acetate, and chromium nitrate to produce thechromium (III) solution.
 4. The method as recited in claim 1, comprisingusing alkaline chromium sulfate as the soluble chromium (III) compoundhaving the following composition in terms of oxides: 20%-45% Cr₂O₃,20%-45% SO₃, 15%-40% Na₂SO₄ 10%-60% H₂O.
 5. The method as recited inclaim 1, comprising mixing in a first reaction vessel, water and thesoluble chromium (III) compound, comprising alkaline chromium sulfate,with each other and then through the addition of an alkaline substance,raising the pH value in the solution and precipitating out chromium(III) hydroxide.
 6. The method as recited in claim 1, comprising mixingin a first reaction vessel, water, zinc oxide, and the soluble chromium(III) compound, comprising alkaline chromium sulfate, with one anotherand then through the addition of zinc oxide or other alkalinesubstances, raising the pH value in the solution and precipitating outchromium (III) hydroxide.
 7. The method as recited in claim 1,comprising filtering out the chromium hydroxide, together with excessZnO that acts as a filtration agent, from sludge and then washing thechromium hydroxide.
 8. The method as recited in claim 7, comprisingreducing a remaining sulfate content through single or multiple washingof the sludge; filtering the sludge and adding a filter cake composed ofzinc oxide and chromium hydroxide to the electrolyte solution for thedeposition of zinc chromium on metallic surfaces.
 9. A use of chromiumhydroxide, which is precipitated from alkaline chromium sulfate byraising the pH value, for the replenishment of electrolytic solutionswith chromium (III) ions.